Probably not too horrible if the child’s mouth can be rinsed out fairly soon (assuming the experience doesn’t send him to the faucet quickly to do just that himself). Some minor chemical burns and irritation from the sodium carbonate and enzymes. It’s unlikely to have lasting ill effects.
“Keep out of reach of children” is doubtless on these packages.
Many decades ago when I was maybe 10, a schoolmate’s smaller sister got under the sink and downed some Dran-O....from what I recall, she was going to have life-long, bigtime problems.